The circadian rhythm is an inborn cycle of biological activity based on 24 hour intervals, and the rhythmicity interacts with sleep homeostasis in order to regulate sleep-wake cycles of individuals. In humans, the circadian rhythm is controlled by the suprachiasmatic nucleus (SCN) (the circadian clock) mediating melatonin secretion from the pineal gland to regulate the sleep-wake cycle. A number of factors, especially daylight, influence the SCN.